I was 17, on death row -- and innocent

Shareef Cousin: I was once youngest person on death row, for a crime I did not commit

He says prosecutors withheld evidence from jury, and he had an alibi

He says after four years on death row, he was exonerated, the prosecutor lightly disciplined

It's not "the worst of the worst" who are sentenced to death, he says -- just the most powerless

Editor's note: Shareef Cousin is a member of Witness to Innocence, a nonprofit organization that supports people who have been exonerated from death row and their loved ones. The opinions expressed in this commentary are solely those of the author. CNN's original series "Death Row Stories" explores America's capital punishment system. Join the conversation about the death penalty at facebook.com/cnn or Twitter @CNNOrigSeries using #DeathRowStories.

(CNN) -- New York Law School professor Robert Blecker was on the radio the other day, telling NPR's "Here and Now" that we should have the death penalty, even if there is a risk that innocent people might be executed. I wonder if Blecker or other death penalty supporters would sacrifice their lives to keep this broken system going.

I was once the youngest person in the U.S. on death row. And I was innocent.

There was videotape of me in 1995, at 16, playing basketball just before the time of a murder in New Orleans' French Quarter. There was even a clock showing the time on the videotape. My basketball coach was driving me and my teammates home at the time of the crime.

My airtight alibi didn't matter.

The eyewitness to the murder, who was on a first date with the victim, told the police she was not wearing her contact lenses or glasses and could only see shapes -- critical information that the prosecution withheld from my attorneys. She was pressured to change her story. At my trial, she identified me as the killer and told the jury, "I will never forget that face."

A detective said two other witnesses identified me as the killer, but it later came out that he had lied to get the arrest warrant. The prosecution pressured another witness into saying he heard me brag about the murder, which was untrue.

As a teenager on death row, I often had nightmares of my own execution. Even though I was innocent, death seemed very close to me, and my mind would light up with images of my being strapped to a gurney, the sound of poison coming from a machine and being pushed through my veins.

Folks who are watching "Death Row Stories" on CNN know that it isn't always clear who is guilty. When I go out to speak to community groups, people are surprised to learn that there are enough people who have been exonerated from death row that we have our own membership organization, Witness to Innocence.

'Existing like an animal'

I'm like, 'Hell no!'

Breakthrough? A police notebook

It is hard to argue that the death penalty is applied fairly. Take it from me, someone who lived alongside guys on death row: The system does not identify and sentence "the worst of the worst" to death -- just the most powerless.

A new study of the backgrounds of 100 people executed in 2012 and 2013 shows that an overwhelming majority suffered from significant mental impairments. Nearly nine out of ten had conditions that made them the same as, or very similar to, people who the U.S. Supreme Court has said are less culpable and constitutionally barred from capital punishment.

I was 17 years old when I was sentenced to death and 20 when I was exonerated in 1999. Six years later, the Court held in Roper v. Simmons that it was unconstitutional to execute offenders aged 17 and younger. The court based its decision, in part, on science showing that the brains of juveniles are still developing. In the study of the last 100 people executed, however, more than one-third committed a capital crime before turning 25 -- the age at which the brain fully matures. Twenty offenders -- one out of five -- hadn't yet reached their 21st birthdays.

The study also found that more than half (54%) of the last 100 people executed had severe mental illness, but the courts did not find them incompetent for execution. One-third had intellectual disabilities, even though the court decided over a decade ago in Atkins v. Virginia that people with "mental retardation" cannot be executed because of their diminished capacity.

At least half the offenders in the study experienced unimaginable abuse as children. Juries may or may not hear about that. Underresourced capital litigators often do not investigate and present all available mitigation evidence.

There is no need to debate people who assert that killing innocent human beings is acceptable or that the death penalty is justified. Those views are fading from the American landscape. In 1999, 20 states carried out executions. In 2013, only nine states did. Six states in as many years have abolished the death penalty. In my lifetime, the U.S. Supreme Court will read the national consensus as turning away from capital punishment and end it once and for all.